In the cultivation of various plant species, numerous structures for housing a growing medium have been proposed to enable the grower to control the quantity of water supplied to the roots of the plant as well as to maintain the integrity of the growing medium. In general, these prior art structures have involved a container for the growing medium and other nutrients together with an irrigation system for supplying water.
In U.S. Pat. No. 5,524,387 to Blake Whisenant, entitled "Plant Cultivation Apparatus and Method", incorporated herein by reference, there is disclosed a reservoir container assembly for the cultivation of plants. The reservoir container in the Whisenant patent comprises a single reservoir container which may be made of solid material such as recycled plastic. The reservoir container assembly includes a growing medium volume defined by the reservoir container which is separated from a drain volume along its lower wall by a permeable partition situated in a spaced relationship above the lower wall. In use, the growing medium volume is filled with a growing medium into which plants are grown. The reservoir container assembly of the Whisenant patent has a top wall made of plastic material such as recycled plastic. The top wall has one or more openings therein for plant growth with the plant growth opening being positioned along the side of the top wall adjacent to the lateral wall.
In the apparatus disclosed in the Whisenant patent, there is at least one drain opening in the lower area of one of the lateral walls to allow excess water to flow out of the drain volume and thereby prevent the level of water in the drain volume from rising above the drain opening. This ensures that the top portion of the drain volume will be filled with air and that the growing medium housed above the permeable partition has contact with the air, such air being important for proper plant growth.
The device of the Whisenant patent also utilizes a column or columns of growing medium that extend into the drain volume at the lower portion of the assembly. The column is filled with growing medium to allow the water in the drain volume to reach from the lower portion of the drain volume into the growing medium volume located above the permeable partition. In use, water will move up the growing medium column and into the growing medium volume by the process of capillary action. In addition, in the device disclosed in the Whisenant patent, the column of growing medium is positioned so that it is adjacent to the lateral wall that is near to the plant opening in the top wall. The Whisenant patent discloses that it is preferable that the columns of growing medium be positioned in the corner of the reservoir container but that they can be positioned anywhere under the lateral wall along which the plants are located. In the Whisenant patent, the single reservoir container and its drain volume area is divided into compartments by rectangularly shaped dividers which may be interconnected with one another. The purpose of the dividers is to ensure that the permeable horizontal partition is positioned in the reservoir container so that the permeable partition lies parallel to the bottom wall and at a given height above the bottom wall thereby forming a drain volume for the water.
The device of the Whisenant patent uses a gradient concept for the growing medium and nutrients. The gradient concept was initiated and evaluated during the 1960's as the nutritional component for a field oriented full-bed mulch system of production. The basic components are a soluble source of nitrogen (N) and potassium (K) on the soil bed surface in conjunction with a continuing water table. The N and K move by diffusion to the root and equilibrate concurrently with the less-soluble nutrients in the soil to maintain a predictable range of decreasing ionic concentrations with associated decreases in the ratio of N and K to total ions in the soil solution. The full-bed mulch minimizes the effects of evaporation and rainfall as physical forces that can alter the ionic composition of the soil solution. The total concept is designed to synchronize the rates of nutrient/water input with those of crop removal, and thus provide long term nutritional stability.
Nutrients in the soil move by diffusion, which is synchronized with removal or move by mass flow with the water which is not synchronized with removal. By eliminating in-bed N-K (conventional procedure) and using on-bed N-K (gradient procedure), it is possible to maintain a continuing nutritional stability in the soil solution.
When conventional nutritional procedures are exposed to variations in the soil-plant-season combinations, nutritional stability in the soil solution can be weakened or destroyed. In the transition to more intensive production systems, conventional nutritional procedures often cannot maintain the nutritional stability required for continuing advances in productivity, whereas the gradient procedure sustains that stability.
Tomato yields from experimental field plots using a gradient-oriented procedure averaged 75 to 80 t/ha for 9 successive crop sessions (1964-68) compared to the commercial average of 16 t from an equivalent (unmulched) ha. In the transition to a full-bed mulch system during the 1970's, average commercial yields more than doubled. In shifting to the full-bed mulch procedure, the recommendations required to sustain the gradient were often diluted with conventional procedure, because change in the prevailing procedures not readily accepted and often is fought with great vigor because it compromises the former investment. The full potential of the gradient generally cannot be attained with an alternative procedure that weakens or destroys the integrity of the concept.
In order to further evaluate the potential of the gradient concept, a surface-applied microsource of water in conjunction with a separate surface source of N and K was used to evaluate a lateral oriented gradient. Tomatoes were grown in containers (wooden boxes) that held 0.4 m.sup.3 of soil that ranged in type from sandy to organic (peat). The maximum yield was about 8 kg/plant (four plants/box) regardless of the soil type. Mass flow nutrients were minimized, but without a water reservoir the variations in moisture jeopardized the nutritional stability.